Natural polyisoprenoid is a generic name of polymers comprising an isoprene unit (C5H8), which are biologically synthesized by some plants and fungi and are typified by natural rubber collected from trees of Hevea brasiliensis. Such natural polyisoprenoids are produced by a number of plants and fungi including Hevea brasiliensis, Ficus elastica, Eucommia ulmoides, and fungi belonging to the genus Lactarius, such as Lactarius volemus (milk mushroom). However, almost none of natural polyisoprenoids other than natural rubber are industrially used. This is because they are insufficient when used as rubber, since most of polyisoprenoids have stereo structures less regular than that of natural rubber, have lower degrees of polymerization and insufficient molecular weights, and are economically difficult to produce in large quantities.
In contrast, the natural rubber can be stably supplied at low cost as a result of long-term investigations and efforts for improving its cultivation method and for stabilizing its quality, and is a rubber material that is adaptable to a widest variety of applications such as tires and other rubber articles. The natural rubber is excellent in rubber elasticity and strength but is insufficient in oil resistance, chemical resistance, heat resistance, and weather resistance. Consequently, it has a limited applicability and is not used as a “high-functional, multi-functional material”, although it is an ecologically excellent material.
Rubber articles for industrial use, especially for use in engine compartments of automobiles, are used in hotter and hotter surroundings. These rubber articles must therefore have higher heat resistance. This is because the temperatures in engine compartments become higher with an increasing fuel economy and for complying with exhaust restrictions. There is an increasing move afoot to clean up the environment and to reduce carbon dioxide which is one of factors inducing global warming. Synthetic rubbers most of which are derived from petroleum raw materials are used in rubber and plastic industries. Carbon dioxide formed upon disposal of these synthetic rubbers becomes a big concern, and a variety of thermoplastic elastomers (TPEs) have been developed and used in recent years. Such TPEs can be reused without disposal. However, they cannot be reused permanently and are discarded ultimately. In addition, even TPEs are not always ideal rubber materials from the viewpoint of global environment, because they are derived from petroleum. Accordingly, plastics prepared from raw materials extracted from plants and having biodegradability have been developed.
Journal of Applied Polymer Science, 66, 1647-1652 (1997) describes a method for producing a hydrogenated product of natural rubber by reacting natural rubber with hydrogen in the presence of a rhodium complex catalyst in an organic solvent at a temperature of 100° C. The investigations in this document, however, are limited to a kinetic investigation of hydrogenation reaction and an investigation of thermal properties of polymer.
In contrast, general-purpose plastics such as polypropylenes (PPs) and polyethylenes (PEs) are widely used in various fields, because they have a wide variety of mechanical properties and moldability, show excellent cost performance, and can be easily recycled. Poly(lactic acid)s and other bioplastics prepared from vegetable materials such as sugarcane and corn have received attention from environmental viewpoints. However, demands have been made on these plastic articles to have further reduced weights and further higher impact resistance and rigidity.
Many attempts have been made to improve impact resistance of resins. Japanese Unexamined Patent Application Publication (JP-A) No. 07-330964, for example, proposes the addition of a styrenic elastomer to an amorphous resin, JP-A No. 11-293058 proposes the addition of an olefinic copolymer to a polypropylene, JP-A No. 61-19652 proposes the addition of a thermoplastic polyurethane to a polyacetal resin, and JP-A No. 2004-143315 proposes the addition of natural rubber to a poly(lactic acid). However, the addition of a styrenic elastomer results in insufficient weather resistance and unsuitable environmental friendliness due to the styrene component. The addition of an olefinic copolymer and that of a thermoplastic polyurethane result in insufficient low-temperature impact resistance. The addition of a natural rubber leads to insufficient weather resistance and impaired appearance of molded articles.
In addition to Hevea brasiliensis, many plants and fungi produce natural polyisoprenoids, as is described above. Effective usage of these plant-derived materials have been studied actively due to increased recent social demands on environmental friendliness. They are expected to be used more and more widely in future along with clarification of biosynthesis mechanisms of natural polyisoprenoids in plants and progress in biotechnologies.
Natural polyisoprenoid latices are collected from trees as latices and are used as raw materials for dipped articles such as gloves, catheters, and condoms, as well as for aqueous paints and coating agents. These latex articles, however, are also insufficient typically in heat resistance and weather resistance, and these properties stand in need of improvements.